Electric heart pacers, commonly called artificial Pacemakers, have been used in patients whose hearts have defective electric conduction systems. An example of such defect is known as heart block where the natural pacemaker on the atrium generates its signal but the signal does not always get through the conductive bundle to stimulate contraction of the ventricle. Although the ventricle may not receive natural signal, it will usually contract ultimately in response to its own escape mechanism. This intrinsic contraction of the ventricle occurs at a lower rate than the atrial contraction, in which case, the patient's blood circulation is inadequate.
Artificial electric heart pacers have been used as a partial remedy for the aforegoing condition. The first generation of pacers were nothing more than fixed rate pulse generators, which connected to the ventricle for causing it to contract at a fixed rate. It was soon observed, however, that on occasion the natural electric signals of the heart were restored and that competition between the natural and artificial pacers occurred, sometimes with undesirable physiological consequences both to patient safety and cardiac efficiency. In U.S. Pat. No. 3,241,556, F. Zacouto, proposed a remedy for this situation in the form of a pacer which only stimulated the ventricle on demand. This patent uses an external electrocardiograph sensor which, in conjunction with suitable amplifying and transmitting means, controls an implanted pulse generator to turn on if a natural beat is delayed or missed and to turn off for a predetermined period after a natural beat occurs. Thus, Whenever the heart misses a natural beat, an artificial stimulating pulse is supplied. Even prior to Zacouto, Davies, in British Pat. No. 826,766 had suggested a pacer which turned on at random after the heart had missed several beats but was inhibited or turned off immediately after a natural beat occurred. Subsequently, Berkovits proposed a scheme in U.S. Pat. No. 3,345,990 for causing the heart to beat at a fixed rate by supplying an artificial stimulating pulse at exactly the same time as the missed natural beat should have occurred.
Among the disadvantages of the aforementioned prior types of pacers is that they compel the heart to beat at a fixed rate regardless of the physiological demands of the body. For example, when the subject is sleeping or involved in low physical activity, the heart rate naturally declines. Prior types of pacers sense the lower natural rate as indicative of missed or seriously delayed beats. The pacing pulse generator then turned on in response to this condition to stimulate the heart at a higher rate than is required by the physiological demands of the body.
This superfluous stimulation results in the subject experiencing an undesirable state of wakefulness and it has other disadvantages. Every unnecessary stimulating pulse consumes electric energy and causes the pulse generator's batteries to deplete sooner. The heart operates less efficiently than it would under the control of its own conductive system. Electrolysis of the heart electrodes is unduly accelerated by unnecessary stimulating pulses. Excess pacing may contribute to cardiac tissue damage.